Conventionally, the level of a liquid in a vessel is detected using a float with a mechanical or magnetic coupling to an external gauge, an ultrasonic or optical transducer which measures time of flight to deduce the liquid level, or a parallel wire capacitance sensor which monitors the change in the dielectric constant between the wires associated with a change in liquid level.
The application of fibre optics to level sensing in liquids is well documented. The principal advantages of this type of level sensor are its passivity, i.e. no moving or mechanical parts, and its intrinsic dielectric properties which mean no spark hazard when used with combustible fluids and virtually zero susceptibility to electromagnetic interference. The more common implementations use small prisms mounted at the end of two optical fibres, a conically shaped optical fibre tip, or a U-shaped bent optical fibre [Canadian Patent No. 1102151](devices of these types are described for example in the article "Experimental Investigations on Fibre Optic Liquid Level Sensors and Refractometers" by K. Spenner et al - IEE OFS 221 pages 96-99). In all instances, the optical fibre probe or sensor is suspended or made to protrude into the vessel and the potentially fragile sensor is exposed to damage by floating debris, vibration, and dynamic effects during filling. The potential for damage is increased if routine maintenance of the sensor is required due to biological or chemical fouling of the optical surface.
The fibre optic sensors just described are exclusively for discrete level measurement, e.g. to sense whether the vessel is empty or full or at some intermediate point. A multiplicity of such point sensors generally represents an impractical configuration for a continuous liquid level measure. A continuous measure is desirable, however, for improved resolution in many applications. Consider the case of an aircraft fuel gauging system. The dielectric properties of the optical fibre sensor are desirable from the point of view of safety with respect to spark hazard and lightning strikes but high resolution and accuracy are also desirable so that excess fuel quantities would not have to be carried, thus reducing aircraft weight and consequently fuel consumption. Present day aircraft fuel level sensors are for the most part capacitance type sensors which lose accuracy when the fuel becomes laden with water and the dielectric constant is changed significantly.
Only one continuous fibre optic liquid level sensor has been found reported thus far ["Fibre Optic Fluid Level Sensor" by M. Belkerdid, N. Ghanderharioun, and B. Brennan in the Proceedings of SPIE Conference 566 Fibre Optic and Laser Sensors III (1985) pages 153-158]. Based on the bending or cladding loss principle, it consists of large loops of a single fibre, the loops being of ever increasing diameter, which are suspended in the liquid. Here again, the sensor packaging is not sufficiently robust for most applications.